Interest in the use of liquid methane, frequently referred to as liquified natural gas (LNG), as a motor vehicle fuel has increased dramatically in recent years. Entire fleets of government and industry vehicles have successfully been convened to natural gas, and some privately-owned vehicles have been converted as well. Congress recently passed an energy bill which would require further increased use of alternative fuels in government and private fleets.
Several factors have influenced this increasing interest in natural gas as a motor vehicle fuel. LNG is relatively inexpensive. It also bums very cleanly, making it much easier for fleets to meet more restrictive pollution emission standards. However, handling LNG remains a significant problem.
An LNG fueling facility typically includes a massive LNG storage tank and a dispensing system. The dispensing system usually relies on a pump to deliver LNG from the massive storage tank to the vehicle. Refrigeration is very expensive. Therefore, insulation around the massive LNG storage tank is relied on exclusively in most installations to maintain methane in a liquid state. Storing and dispensing LNG from an insulated tank poses several problems.
LNG is preferably kept in a saturated state in the massive storage tank and as it is pumped through the dispensing system. Otherwise, heterogeneous phase methane is dispensed into a vehicle, which is undesirable. First, a vehicle's tank is only partially filled with usable fuel, reducing the range of the vehicle. The time between vehicle refuelings falls and this places an increased burden on the limited capacity of an LNG fueling facility to service a fleet. And second, obtaining an accurate measure of the amount of LNG actually dispensed into a vehicle's tank is not possible using conventional mass flow meters. The LNG fueling facility therefore cannot accurately charge for the LNG dispensed, which is especially important for facilities intended to service multiple fleets or individual consumers.
Pressure within the massive storage tank must also be kept below a maximum allowed pressure for safety. It is physically impossible to insulate a tank for no heat transfer. Therefore, heat from the environment continually warms the liquid methane. Once the temperature of the liquid methane rises above its saturation temperature the pressure under which the liquid is placed, the liquid methane boils, trapping the vapor in the tank. The liquid methane continues to boil off vapor, raising the pressure in the tank until of the pressure on the liquid methane reaches saturation pressure for the temperature of the liquid. Additional volume made available from to dispensing of LNG relieves some pressure. However, if the pressure within the tank meets or exceeds a maximum safe pressure, it must be vented in a procedure colloquially referred to as "blowing down the tank".
Blowing down a tank is undesirable. Releasing methane into the atmosphere can create a potential for explosion and is an environmental hazard. Although conditions which surround venting can be carefully controlled to minimize risks, releasing methane into the atmosphere is preferably avoided.
More importantly, taking the pressure off the liquid may lower its saturation temperature below its actual temperature, causing the liquid to boil. Blowing down the tank thus results in boiling, with the methane coming out of a homogenous liquid phase and assuming a heterogeneous phase. Blowing down the tank, however, dispels heat from the tank and results in lowering the liquid temperature. Less pressure is thus required to maintain the methane in a saturated liquid phase after blow down. Nevertheless, it is still desirable to slightly "sub-cool" the liquid methane by passing some liquid through a heat exchanger to vaporize it and returning the vapor back to the tank to pressurize and compress the liquid to raise its saturation temperature. Thus, some heat is returned to the gas occupying the void in the tank above the liquid level.
Specially trained operators are usually required to maintain the facility and to dispense the LNG. Having to employ specially trained operators to handle the LNG and cryogenic fluids not only makes LNG fueling stations more costly, it also makes them generally less appealing to fleet operators and particularly unappealing to average drivers who service their own automobiles. However, even specially trained operators are sometimes unable to properly condition the tank.